Windings for power-transformer cores have traditionally been formed by winding a thin, flat strip of magnetic material into a circular pattern, and then pressing the wound material into a rectangular pattern. The rectangularly-shaped material is then annealed to relieve the internal stresses induced by the pressing operation (high internal stresses within the winding can induce operating losses, and are therefore undesirable).
Alternatively, a wound core can be formed by bending and then stacking a plurality of laminae formed from a magnetic material. More particularly, substantially flat laminae of various lengths are each formed into a U-shaped pattern by bending each laminae at two or more locations thereon. The U-shaped laminae are then stacked, i.e., superposed, and bound to each other in order of decreasing size. In other words, the laminae are arranged one-inside-the-other. This arrangement forms a U-shaped laminate. A second U-shaped laminate of complementary shape and size is formed in a similar manner, inverted, and fixed to the first U-shaped laminate to form a rectangular winding.
The laminae are typically bent by using a die set 100 comprising a male die 112 and a female die 114, as shown in FIG. 5. The male die 112 comprises a v-shaped end portion 112a. The female die 114 defines a v-shaped groove 115 adapted to receive the end portion 112a. A lamina 116 is placed between the end portion 112a and the groove 115. More particularly, a desired bending location on a lamina 116 is aligned with the end portion 112a and the groove 115. The male die 112 is subsequently driven downward, toward the female die 114. The resulting contact between the end portion 112a and the lamina 116 drives the desired bending location on the lamina 116 downward, into the groove 115, and thereby places a bend in the lamina 116.
Forming a transformer-core winding by bending and stacking a plurality the laminae offers advantages in relation to forming the winding from a continuous strip of material. For example, the operating losses associated with the laminated winding, without annealing, are comparable to those associated with the continuously-wound, annealed winding (provided the plastic deformation at the bending locations on each lamina 116 is limited to approximately five times the thickness of the lamina 116). Eliminating the need for the annealing process substantially reduces the time and expense needed to manufacture a transformer-core winding.
The bending process described above, however, presents certain disadvantages. For example, the configuration of the die set 100 causes the ends of the lamina 116 to deflect upwardly as the lamina 116 is bent. More specifically, the downward movement of the lamina 116 at the bending location, in conjunction with the resistance offered by the female die 114, causes the ends of the lamina 116 to be displaced upwardly, as shown in FIG. 5. The lamina 116 is also drawn inward, toward the dies 112, 114. This displacement decreases the precision with which the 116 can be bent. In particular, the noted displacement of the lamina 116 makes it more difficult to maintain the initial alignment between the male die 112 and the desired bending location on the lamina 116 as the lamina 116 is bent.
The imprecision introduced by the displacement of the laminae 116 during the bending process increases the manufacturing tolerances of the laminated winding (and a transformer core formed therefrom). The operating losses of a transformer core generally increase with increasing manufacturing tolerances. Hence, higher manufacturing tolerances are generally undesirable. Furthermore, the above-described bending process is unsuitable for placing multiple bends in the lamina 116 due to the noted movement of the lamina 116. (The ability to place multiple bends in the lamina 116 on a simultaneous basis can substantially reduce the time needed to manufacture a laminated winding, and is therefore desirable.)
An ongoing need therefore exists for a process for bending laminae for a transformer that locates the bend location with relatively high degree of precision, and that is suitable for forming multiple bends on a simultaneous basis.